Background: Acquired PARP inhibitor (PARPi) resistance in high-grade serous ovarian cancer (HGSOC) as a result of restored homologous recombination has been observed following secondary mutations that restore full-length protein in BRCA1, BRCA2, RAD51C, and RAD51D. Additionally, loss of BRCA1 methylation has also been shown to confer resistance. However, little is known about the role of RAD51C methylation in acquired PARPi resistance. In ARIEL2 Part 1, a phase 2 study of the PARPi rucaparib in ovarian carcinoma, four (2%) tumors demonstrated RAD51C methylation. The present study utilizes HGSOC patient derived xenografts (PDXs) and recurrent samples from ARIEL2 to assess the role of RAD51C methylation in the development of PARPi resistance.

Methods: To drive PARPi resistance, PDX039, an extremely PARPi-sensitive model lacking demonstrable mutations in DNA repair genes, was treated cyclically with niraparib (100 mg/kg) for 21 days, after which the tumor was allowed to regrow and re-established in new mice for the next treatment round. To evaluate the frequency of methylation change, RAD51C methylation was analyzed in 12 rucaparib-treated mice (300 or 450 mg/kg) harboring PDX183, a PARPi-sensitive model without mutations in DNA repair genes. Global changes in gene expression following development of PARPi resistance were assessed by RNA sequencing. RAD51C promoter methylation was evaluated by bisulfite sequencing. Subsequent functional analysis included qRT-PCR, IHC, and western blot. DNA damage response pathways are being evaluated by immunofluorescence ex vivo following niraparib, rucaparib, or IR.

Results: PDX039 grew through PARPi treatment by the third and fourth cycle of therapy. RAD51C was the only DNA repair gene to show significant change in RNAseq analysis (log2 fold-change=8.43; p=2e-192), corresponding with a loss of RAD51C methylation. Moreover, after just one round of PARPi treatment, RAD51C methylation was lost in 1 of 12 PARPi-treated PDX183 xenografts. RAD51C methylation loss ultimately resulted in restoration of expression, for which functional analysis is ongoing. Analysis of patient samples is currently underway.

Conclusions: In HGSOC PDX models, RAD51C methylation affords PARPi sensitivity in the absence of DNA repair gene mutations. Treatment pressure with PARPi can reverse RAD51C methylation and restore RAD51C expression. Isolated changes in methylation of the RAD51C locus are sufficient to restore HR and convey PARPi resistance.

Citation Format: Rachel M. Hurley, Ksenija Nesic, Cordelia McGehee, Olga Kondrashova, Maria I. Harrell, Paula A. Schneider, Xiaonan Hou, Cristina Correia, Karen S. Flatten, Giada V. Zapparoli, Alexander Dobrovic, Kevin K. Lin, Thomas C. Harding, Andrea E. Wahner Hendrickson, Elizabeth M. Swisher, Matthew Wakefield, S. John Weroha, Clare L. Scott, Scott H. Kaufmann. Loss of RAD51C promoter hypermethylation confers PARP inhibitor resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5885.